1. Field of the Invention
This invention relates in general to semiconductor devices and more particularly to devices implemented with semiconductor on insulator (SOI) technology.
2. Description of the Related Art
The surface orientation of a semiconductor material is descriptive of the lattice orientation of the material at the surface of the material. With some semiconductor circuits, it may be desirable to implement the P-channel transistors and N-channel transistors in semiconductor active regions having different surface orientations. For example, some N-channel transistors have a relatively higher electron mobility in silicon having a surface orientation (100) as opposed to the electron mobility of an N-channel transistor in silicon with a surface orientation (110). On the other hand, some P-channel transistors will have a higher hole mobility with their channels being implemented in silicon having a surface orientation (110) as opposed to silicon having a surface orientation (100).
Prior semiconductor devices have had semiconductor on insulator (SOI) configurations where the active semiconductor area for the N-channel transistors has a surface orientation of (100) and the active semiconductor area for the P-channel transistors has a surface orientation of (110). The different surface orientations are formed by removing areas of the active silicon layer having a first orientation (e.g. (100)) and removing the underlying oxide in those areas to expose a silicon substrate having a second surface orientation (e.g. (110)). Silicon is selectively expitaxially grown in the exposed areas such that the expitaxially grown silicon has the same surface orientation as the substrate. Oxygen is then implanted into the epitaxially grown silicon (and the wafer subsequently annealed) to form an insulator layer between the epitaxially grown silicon and the substrate. However, the selectively epitaxially grown silicon may include defects in the subsequently epitaxially grown silicon active layer e.g. at a boundary of the epitaxially grown silicon. Also, maintaining a clean surface on which to selectively grow the epitaxial silicon may be difficult due to different materials on the wafer and their pattern densities. Also, selective epitaxially grown silicon processes may lead to non epitaxial silicon growth on unwanted areas (e.g. on dielectrics). Furthermore, the insulator formation in the epitaxially grown silicon may generate defects in the epitaxially grown silicon active layer.
What is desirable is an improved process for forming a semiconductor device having an SOI configuration with active layers having different surface orientations.
The use of the same reference symbols in different drawings indicates identical items unless otherwise noted. The features shown in the Figures are not necessarily drawn to scale.